In recent years reflective-transparent solar control film has achieved an important place in the economy of the United States and several other countries. Products of this type commonly involve a 1-mil biaxially oriented polyethylene terephthalate film, having a vapor-deposited aluminum layer about 25-125 microns thick on one face. The aluminum layer is contacted and protectively covered by a thin, transparent water-insoluble moisture-transmitting barrier coat. As shown in Antonson and Berger U.S. Pat. No. 3,290,203, a water-activatable adhesive is used to bond the thus-protected aluminum surface of the film to the inside of a windowpane, the water which activates the adhesive slowly escaping through the barrier coat, aluminum layer and base film over a period of a few days.
Reference is directed to the aforementioned Antonson and Berger patent for a description of how laminated windowpanes formed in this manner serve to substantially reduce heat transmission into the room on which the window opens without significantly reducing light transmission. Laminated windowpanes of this type can be formed by applying a water-soluble adhesive to either the glass or the protectively coated surface of the film prior to lamination. In either case, the slippery nature of the adhesive permits sliding the film to position it precisely against the glass. This feature is particularly important when it is considered that the film applied to a given windowpane may easily be 4 feet wide and 6 feet long.
Despite the commercial success achieved by solar control films of the type described above, conditions are sometimes encountered where such products do not have the useful life which would be desirable. This is particularly true when the interior surface of the laminated windowpane is exposed to extremely high humidity, especially when condensation or frost formation results. In such conditions, the film sometimes loosens, resulting in the appearance of unattractive "fingers" where the film pulls away from the glass, reducing the effectiveness of the film and greatly marring the esthetic qualtities of the laminated windowpane. Even so, complete removal of the film under normal conditions can be accomplished only after subjecting it to steam, applying wet newspapers for an hour or so, etc.
Palmquist and Erwin U.S. Pat. 3,152,950 discloses an opaque heavily metallized polyester film in which a normally tacky and pressure-sensitive adhesive layer is applied over the metal coating. It might be supposed that the water-activatable adhesive employed in the aforementioned Antonson and Berger patent could be replaced with a normally tacky and pressure-sensitive adhesive of the type used by Palmquist and Erwin, but doing do is fraught with serious problems. Although it is easy enough to unwind, sever, and apply a length of comparatively narrow pressure-sensitive adhesive tape, it is virtually impossible to work with a large area of such a product.
To handle a large piece of pressure-sensitive adhesive sheet material at all, it is necessary to employ a removable release liner which protects the adhesive until the sheet material is to be applied but which also increases the cost of the product. Even after the liner is removed, the large sheet of material tends to stick to itself, to become impressed with the fingerprints of applying personnel, to pick up lint, etc., all of which disfigures a solar control film and destroys one of the main requirements for its acceptability. Further, the pressure-sensitive nature of the adhesive is such that it is difficult to reposition a sheet of film once it is placed in contact with the glass. Unless great care is exercised, this difficulty in positioning exists even when water or detergent-containing water is applied to the windowpane, the pressure-sensitive adhesive, or both, prior to installation.
It has heretofore been suggested that small decals could be provided with a tacky adhesive, temporarily inactivated by a water-soluble coating, for mounting on the object to which the decal is ultimately to be applied. U.S. Pat. 2,641,562, for example, teaches that the decal can be immersed in water, immediately dissolving the water-soluble coating, and then applied directly to the appropriate substrate. .[.Although it might be supposed that a solar control film could similarly be provided with a pressure-sensitive adhesive temporarily inactivated by a water-soluble coating, those skilled in the art have been discouraged from doing so..].
.[.It is simple enough to immerse a decal in water and thus remove a water-soluble coating, but the problem is far more complicated when a solar control film of considerable area is to be handled. Washing the coating from a 4-foot .times. 6-foot piece of film is difficult, and, even if such is accomplished,.]..Iadd.Such teaching would serve to discourage one skilled in the solar control film art from trying to use a water soluble coating over a tacky adhesive on a solar control film, since once the water soluble coating is washed off and the tacky coating is exposed .Iaddend.the resultant product possesses the difficult handling characteristics previously attached to pressure-sensitive solar control film products. .[.It.]. .Iadd.On the other hand, it .Iaddend.is reasonable to suppose that failure to completely remove the water-soluble coating would prevent the pressure-sensitive adhesive from firmly bonding the solar control film to the windowpane. Indeed, it has been found that when a .[.decal.]. .Iadd.solar control film .Iaddend.of the type just described is subjected to a stream of running water without completely removing the water-soluble coating, and the thus-washed product allowed to dry, the pressure-sensitive adhesive is still so effectively masked that it displays only very mild adhesion.[.. Although adequate for decals, which are extremely thin and small, such adhesion.]. .Iadd.which .Iaddend.is entirely inadequate for use with solar control film. Further, the washed and dried product has hazy, blurred, non-uniform and otherwise unsatisfactory optical properties which point away from any use on a windowpane.
Special problems are also involved in the use of pressure-sensitive adhesives in the presence of sunlight. Many such adhesives crosslink and/or discolor when exposed to ultraviolet light. Still other adhesives do not age or discolor but bond so firmly to glass that any attempt to remove sheet material therefrom leaves the adhesive bonded to the glass.
Thus, prior to the present invention, it is believed that there has never existed a moisture-resistant transparent-reflective solar control film which can be readily handled, easily installed in large pieces on a window, easily positioned, quickly adhered, and readily removed.